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How to install a heat pump into an existing property

Posted by John Barker-Brown on 21 February 2011 at 9:18 am

As the advantages of heat pumps become better known and with the expected Renewable Heat Incentive, more people are looking at whether the technology can be applied to their existing properties.

The advantages of installing a heat pump in a new build property is well documented. It is also possible to install heat pumps into existing properties, but there are various considerations which which determine how well, and even if, a heat pump will work effectively.

1. Sizing: the heat loss of the property is determined by how well the building is insulated. If the insulation is not known then it becomes difficult to size the heat pump accurately. The only way this can be achieved with any degree of accuracy is to commission a Standard Assessment Procedure report (SAP) which takes into account the build, insulation, heating system, etc and can be used to determine a peak heat load.

2. Insulation: This plays a big part in how effectively a heat pump operates. As the heat pump produces a lower flow temperature than traditional fossil fuel systems, the amount of heat it transfers to a room is limited (for a given heat emitter area) and is less than the traditional boiler.

If you raise the heat pump outlet temperature to its maximum (approx 50 degrees C) the heat pump has to work harder and its efficiency decreases. The coefficient of performance (CoP) at 50C is now only 3 as opposed to 4 (at 35C).

You also need to think about whether the actual heat emitting device, i.e. radiators, underfloor, etc, will output enough heat at the heat pump’s lower flow temperatures, to keep the building warm enough to be comfortable, particularly when the temperature outside is cold.

Reducing the energy requirement for any building should be a central theme to the design process. Any investment in an upgraded insulation specification will have a far swifter payback than the return on any renewable technology. For this reason, you should consider improving the insulation level beyond the minimum requirements stipulated in the Building Regulations.

If your building is very poorly insulated, the low temperature output from the heat pump may mean that it will never get warm and that the running costs for the heat pump are expensive. Possibly even higher than the existing fuel choice, particular if it is mains gas. We do not recommend installing a heat pump in a poorly insulated building.

3. The existing heating system: many existing properties will be heated by radiators. As radiators have a smaller surface area than underfloor heating, they need a higher flow temperature to provide heat. Before a radiator will provide any heat the flow temperature needs to be at least 45oC. This means that the efficiency of the heat pump is reduced and the Coefficient of Performance (CoP) drops from 4 (for underfloor heating in screed) to 3. This is a drop in efficiency of approximately 25% (and will result in higher electricity bills).

Due to the lower flow temperatures produced by a heat pump compared to a traditional fossil fuel boiler, the surface area of the radiator may need to be increased to provide the required heat into the building. Therefore all radiators will need to be oversized.

However, there are a number of situations which suit a radiator system, such as Social Housing schemes and clients may well accept the lower efficiencies of a radiator system for the ease of installation benefits, quicker response than underfloor heating, etc.

4. Land Available: This criteria only applies for ground source heat pumps, which uses renewable stored solar energy from the ground to heat the property; it absorbs this energy by means of a ground array buried within the property’s boundaries.

It is important to make sure that the correct amount of pipe for the application is buried and that it is buried correctly. If not enough pipe is installed then the ground could potentially run out of energy mid heating season, leaving you cold and without heat.

It is important to remember that if the heat pump is producing domestic hot water as well as space heating, more ground array is needed, as there is an additional all year round load on the ground. As a guide roughly 2 to 2.5 times the area being heated is required to install horizontal ground arrays.

If you don't have enough land available an alternative to horizontally laid ground arrays is a vertical drilled borehole. These can be down to a depth of over 100m. Drilling a borehole is a specialist activity and as such can be expensive. As a guide these costs can be £40-£45 per m.

Any existing building where a heat pump is proposed needs to be looked at very carefully as a heat pump is not always the correct choice.

Photo: Kensa

About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.

If you have a question about anything in the above blog, please ask it in the comments section below.

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12 comments - read them below or add one


RichYComment left on: 29 December 2014 at 12:40 am


I see the installation costs for an air source heat pump estimated at about £5,000 - £10,000. On the face of it that seems a lot. Could anyone explain what you get for the money?

I'm about to install underfloor heating, as a DIY project, to my ground floor so an air to water heat pump would be an ideal if it weren't for the installation costs.

I see the components cost about £2,000 - £3,000. The installation appears to be:
- Stick a box on the outside of the house
- Run pipes into the house
- Perhaps install an appliance inside the house too, if it is a split system
- Connect to the heat sink/hot water storage

On the face of it that appears to be a day's work, so I'm wondering what the other £4,000 - £9,000 goes towards. Is this based on installing a pump and the cousehold components that will benefit from the pump?

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Cathy Debenham

Cathy DebenhamComment left on: 30 August 2013 at 9:48 am

Hi Jeremy - it's good to hear positive stories. I'm glad your heat pump is keeping you warm and keeping the bills down.

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JeremyOlmComment left on: 30 August 2013 at 8:44 am

We moved into an old converted barn in Braintree with no heating and, after doing a lot of research into heating options, we contacted a reliable and trustworthy company we found online who had a very good track record. They came and installed a heat pump into the property and we have had no regrets. The property is very old and situated in the north of England so as you can imagine it gets very cold at certain, more often than not actually, times of the year! We are also pleased to see that our heating bills are reasonable for the size of the place.

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John Barker-Brown

John Barker-Brown from Kensa Heat PumpsComment left on: 22 May 2013 at 10:20 am

Hi andall,

With regards to the ground loops for a ground source heat pump, the more loops in the ground means the heat extraction from the ground is spread over a larger area. This results in a higher ground temperature and hence a marginal improvement in efficiency of the heat pump and doesn’t over extract the ground. The only issue is that to get the best heat transfer from a fluid flow, it needs to be what is classed as turbulent flow with a Reynolds number above 2500. The more ground loops means the flow per loop is decreased and hence the turbulence drops.

The number of loops where this laminar flow (not turbulent) occurs depends on each individual system. A MCS installer is required to check the Reynolds number by calculation for the installation and confirm it is above 2500 so should be able to tell at which point the number of loops becomes an issue. 

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andallComment left on: 22 May 2013 at 9:17 am

I have read lots of 'advice' regarding GSHP ground loops, and the importance of getting the correct size/area. Am I right in thinking it's impossible to oversize a ground loop?

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dlhComment left on: 10 December 2012 at 12:28 pm

Thank you Sims Solar.  I will pursue section 2.3.2, but think that success will depend on demonstrating that I am NOT getting a new central heating system! 

As I understand it, if I install a new central heating system, it is 20% VAT, with some [substantial] componants at 5%.  If however the job is defined as substituting a new heat source into an existing system, then I might be in with a chance at having the radiators seen as an 'ancillary supply' to a job rated at 5%.

How much of a central heating system can be replaced without it becoming a new system?  The new radiators will be in 2 rooms out of the 10 currently heated by a system driven by LPG.  The 2 rooms are large, constituting around a third of the cubic volume of the house.  The radiators in the other 8 rooms, and their pipework, will remain unchanged.

Re your question about ground source heating being the most appropriate - I am not certain what alternatives you might think I have.  Ground source heating should save on the order of £2000 per year over the present LPG heating.  The property is indeed hard to insulate, although we have done what is economic without changing the internal and external appearance.  Given that we want warmth, what competing choice might we have?

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Sims Solar Ltd

Sims Solar LtdComment left on: 4 December 2012 at 11:17 pm

Dlh, interesting one. Looking at the details provided my initial observation is that your reference to clause 3.3 refers to grant aided installations, is this the case? Reference is made to 2.3.3. but I would have thought you are better of pursing clause 2.3.2 which talks about "An ancillary supply is a supply of goods or services that is a better means of enjoying the principal supply" in this case the gshp is the principal supply and the larger radiators a better means to benefit from the gshp. only a thought. As an installer of reduced rate items I can understand how the installer does not want to get the VAT rate wrong.

You could give the VAT man a call, see what he says, you may have to write to him.

I've been querying the VAT office about the correct rate of VAT on devices like the immerSUN. I have in writing it should be 20% in all situations, I continue to challenge them on this.

I'm going to to throw a curve ball in here, is heat pump technology the most appropriate technology for a property as old as yours and inherently poorly insulated?

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dlhComment left on: 3 December 2012 at 6:21 pm

I have had a quotation for ground source heating, retrofitted to a stone and brick house built in 1890.  The quotation came in two parts, one for the bore holes and the heat pump, and one for replacing the radiators in part of the house with ones of higher capacity.  The company did a room by room analysis of heat requirements and existing radiator capacity, and concluded that part of the house needed significantly higher radiator capacity to meet the requirements that the heat pump provide 100% of the reasonable heat needs.

I have no quarrel with this conclusion.  They are spot on.  The issue is this: the bore holes and heat pump will attract 5% VAT, while the replacement radiators and higher capacity copper pipes are rated at 20%, adding nearly £1000 to the total job.

I queried this, citing HMRC Notice 708/6 (November 2011) as saying that the whole installation should be at 5% VAT.   Section 3.3 says

3.3 Central heating systems

The reduced rate applies to the installation, repair and maintenance of a boiler, radiators, pipework and controls forming a central heating system.

This includes micro combined heat and power systems, which are heating systems that also generate electricity.

The reduced rate includes repairs and replacements of such equipment, whether or not the original system was installed under a relevant grant-funded scheme.

My view is that the new radiators and pipework are clearly an essential part of the energy-reducing heat-pump installation, as without them, the system would fail to deliver as legally required. 

The company checked with their VAT advisor and confirmed that VAT is at the standard rate of 20%, citing HMRC 708/6  2.3.3

2.3.3 Installation of energy-saving materials with other goods and services

Sometimes when individual goods and services are provided together, there is not a single dominant supply and so the individual goods and services supplied together have equal importance, often taking the form of something else. For example, a central heating system may consist of a conventional boiler, radiators, copper pipe, radiator values, heating controls etc. Supplied together, they form a single supply of a central heating system.

While some components of the central heating system may be reduced rated if supplied on their own, here they are part of a wider supply of a central heating system and since a whole central heating system is not included in the list of energy saving materials eligible for the reduced rate (see paragraph 2.5) the whole supply is standard-rated.

Question - has anyone else encountered this, and is there any ruling or precedent that applies?


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John Barker-Brown

John Barker-Brown from Kensa Heat PumpsComment left on: 13 July 2011 at 10:39 am

Dear MrsN,

Thanks for the comments. I'm glad you find the blogs useful.

With any heat pump, insulation is the key and we would always recommend that the maximum amount of insulation is fitted. Without insulation the heat pump has to operate at a higher temperature and becomes less renewable as it uses more electricity. In the worse case you could end up with high energy bills and cold rooms. For any heat pump insulation we would recommend levels of insulation to at least current Building Regs.

If you are considering an air source make sure that the unit will provide the correct output at the higher internal temperature and low ambient temperature outside. A number of manufacturers state the outputs at 35C internal and 7C external. Deviation from these conditions will decrease the output of the unit.

You will also need planning permission for the air source and take into consideration defrost cycles, which either use direct immersion or take heat out of the building (from a buffer vessel).

With regards to running oil upstairs and a heat pump downstairs, it is possible to do this, however you need to ensure that the heating systems are completely separate (otherwise with its high return temperature the oil boiler will take all the load, and the heat pump will never turn on) and both heating systems are operated at the same time. If they aren't then the air source needs to be sized for the whole property as it will try and heat the whole building.

Finally with regards to the underfloor and running this first on oil, just make sure that the underfloor is designed for a heat pump. This generally means closer centres on the pipes due to the lower flow temperature from the heat pump. You might also need to close any mixing valves on the system as for a heat pump these are not required but are generally fitted for an oil boiler.

John Lightfoot who also writes on this website is a good contact for air source. My company Kensa only manufacture ground source as we feel this is a better solution.

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MrsNComment left on: 12 July 2011 at 7:58 pm

Hi John,
Can I ask some really basic questions?  I have read the blogs here which are very helpful & so far so good.  We have just had an offer accepted on a mid-1800s cottage needing complete renovation which currently runs oil fired CH/HW & I am desperate to reduce our dependence on this.  There is also a damp problem as the floors consist of bricks & then earth so we have been advised to dig out put in concrete screed/insulation etc.  So, the obvious answer seems to be underfloor heating throughout ground floor with an air source heat pump.  Upstairs and HW would still run on oil.  QUESTIONS:
Does this sound all sound logical & feasible to you?
In terms of the 'sufficient insulation' required would good loft insulation, secondary glazing & brick walls be likely to be sufficient?
Lastly, as we probably can't afford to do all the work/secondary glazing in one go, have you ever heard of underfloor heating being fitted & running off oil initially then an air source heat pump being retro-fitted to the system a year later once all the insulation was in place?

Thanks for your advice & sorry for long-winded questions!

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Kensa Heat Pumps

Kensa Heat PumpsComment left on: 23 June 2011 at 2:50 pm

Hi Bob,

Thanks for the comment. Its interesting to see that you have found what we have always thought written in a book! Our general stance with clients now is for existing installs either to

a) if they haven't yet installed, reduce their boiler temperature to around 50C and see if the radiators provide enough heat.

b) if they have committed to a heat pump, leave the existing radiators and only replace if the room feels cold.

c) have a radiator survey done.

A number of our clients are finding that they are warm enough on their existing radiators, simply due to the original heating engineer oversizing them.

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Bob Irving

Bob IrvingComment left on: 23 June 2011 at 10:06 am

In my research into heat pumps, I found a comment in one of the older books "Heat pump" by McMullan - there are hardly any new ones! - that radiators are likely to be oversized on existing systems. I quote

"Boilers are cheap, and it is customary to install quite large boilers in quite small buildings, to ensure that there will always be enough heat. As an example, a house of 100 m2, which in typical cold winter weather might require 5-7 kW of heating, might easily be fitted with a boiler of 18 kW ... output. This means that enough radiator area has to be provided to emit 18 kW at reasonable water temperatures. ... Consequently it is necessary to install radiators with enough area to dissipate only about 7 kW at 50/60°C, which by a happy chance is almost exactly 40% of 18 kW." (McMullen, 1981, p107) 

This implies that (probably older) radiators already installed in houses would be sufficiently large enough to provide enough output from a hp at 45-55 degrees. Does anyone have any experience of this?

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